Continuously Growing Ultrathick CrN Coating to Achieve High Load

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Continuously Growing Ultra-Thick CrN Coating to Achieve High Load Bearing Capacity and Good Tribological Property Zechao Li, Yongxin Wang, Xiaoying Cheng, Zhixiang Zeng, Jinlong Li, Xia Lu, Liping Wang, and Qunji Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16426 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Continuously Growing Ultra-Thick CrN Coating to Achieve High Load Bearing Capacity and Good Tribological Property Zechao Li1, 2, Yongxin Wang1,∗, Xiaoying Cheng2, Zhixiang Zeng1, Jinlong Li1, Xia Lu1, Liping Wang1，Qunji Xue1 1. Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China; 2. Institute of Materials, Shanghai University, Shanghai 200072, China; Abstract:

Continuous growth of traditional monolayer CrN coatings up to 24 h is successfully achieved to fabricate ultra-thickness up to 80 µm on the 316 stainless steel substrate using multi-Arc ion plating technique. The microstructures, mechanical properties and tribological properties evolution with the CrN coating continuously growing was evaluated in detail. TEM observations and inverse Fourier-filtered

∗

Corresponding author: Tel.: +86 0574 86697306. E-mail address: [email protected] (Y. Wang).

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images reveal a relaxation mechanism during continuous growth of CrN coating, which can lead to decrease the residual stress when coating growth time exceeds 5 hours. The scratch test and friction test results both show that the load bearing capacity of coating has been significantly increased as CrN coatings growing thicker. During the scratch test, the ultra-thick CrN coating with thickness of 80.6 µm is not failed under load of 180 N and the dominant failure mechanism is cohesive failure including wedge spallation and cracking. The dry-sliding friction test results show the mean coefficient of friction and wear rate of ultra-thick CrN are respectively decreased by 17.2% and 56.8% at most compared with the thin coating (thickness is 5.4µm). The ultra high load bearing capacity and excellent tribological property are attributed to the relaxation mechanism and limited contact pressure as the coating continuously growing. Keywords: ultra-thick CrN coating; relaxation mechanism; load bearing capacity; scratch test, tribological performance 1. Introduction The coatings, which fabricated by physical vapor deposition (PVD) technology such as chromium nitride (CrN), titanium nitride (TiN), zirconium nitride (ZrN), vanadium nitride (VN), Diamond-like carbon (DLC), Graphite-like carbon (GLC) and various composites based above-mentioned (CrAlN, CrSiN, CrCN, CrAlSiN, TiAlN, TiSiN, TiCN, TiAlSiN, CrTiAlN, ZrCN, VAlN, Ti-DLC, Si-DLC, WC-DLC, Cr-GLC, etc.) with good properties like high wear resistance, corrosion protection and low friction coefficient under variety of conditions have become more and more


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popular in the field of material protection such as drills, gears, bearings, molds, medical implants, seals and instruments1-5. Nowadays, the thickness of the coatings is an important parameter having drawn considerable interest in these applications6-9. For the traditional thin coatings, low load bearing capacity, low lifetime and damages quickly reach down to the substrate have restricted the development of PVD coatings in more and more severe mechanical application conditions such as high working temperatures, high humidity, high salt spray and high work load carrying10-13. However, most PVD coating generally are limited the small thickness because of the mismatch in the chemical bonding between the coating and substrate, high interfacial free energy, ion-peening mechanism and the disbanding problem14-17. Thus, the design and development of thicker PVD protective coatings are most desired and important in fundamental research and industrial applications. In recent years, many researchers have fabricated the wide variety of PVD coatings with high thickness based on the two design concepts of doping C, Si, W, Cr, Al and other elements and fabricating multilayer coatings8, 18, 19. For example, the thick silicon multilayer (20 µm) with alternate compressive / tensile stress layer pairs was designed and deposited by the Yong Yang et al20. And Hsiu-MinLin et al. also have deposited the thick-layered nanocomposite Ti–Si–C–N coatings21. C. Maurer et al. fabricated 30 µm thick multilayer (Ti/TiN) coatings by magnetron sputtering22. Comparing with above PVD coatings, the chromium nitride (CrN) coatings not only have high hardness, good wear resistance and chemical inertness as well as the superior oxidation resistance up to 700 °C, but also can relatively easy achieve



relatively high thickness without the stress problem, which have been rapidly developed by some PVD techniques in recent years including arc ion plating (AIP), ion beam assisted deposition (IBAD), magnetron sputtering (MS) and so on1-4, 11, 13, 23. Lin et al. has successfully fabricated thick and dense CrN/AlN superlattice coatings (up to 10 µm) with good adhesion and superhardness by modulated pulse power (MPP) magnetron sputtering technique24. Compared with other deposition technologies, arc ion plating (AIP) technology is an outstanding method and preferred by industrial produce for hard coating due to the high ionization ratio, high deposition rate, flexibility of target arrangements and the merits of producing coating both with good coating-substrate adhesion25,

26

. Lei Shan et al. reported that Cr/Cr2N/CrN

multilayer coating synthesized by multi-Arc ion plating with a thickness of 24.4 µm exhibited good anti-corrosion and tribological performances27. Shanhong Wan et al. has successfully fabricated CrN/GLC coating on GCr15 steel with total thickness of about 38 µm28. These unique interface architectures demonstrate longer lifetime as well as improved mechanical properties for the thicker CrN based coatings. However, these approaches are not always commercial uses because the local compositional and microstructural ununiformity within coatings. In this present study, the continuous growth of traditional monolayer CrN coatings up to 24h to fabricated ultra-thick were successfully achieved on the 316 stainless steel substrate using multi-arc ion plating technique, which is a simple but effective strategy to improve load bearing capacities and tribological behavior. And the transformations of the microstructure, mechanical


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properties and tribological behavior with the growth of CrN coating were systemic investigated. The present study is rather unusual relative to the current thinking for PVD coating interface architectures routes and the aim of this study is to lay foundation for the potential application of various ultra-thick nitride coating in fundamental research and industrial applications in potential engineering. 2. Experimental details 2.1. The coating depositions The CrN coatings with different continuous growth time were fabricated on 316L stainless steel (30 mm×20 mm×2 mm) substrates and silicon substrates by multi-arc ion plating system (Hauzer Flexicoat 850) with the chromium targets (purity ＞ 99.5 wt%, Φ 63 mm). Prior to the deposition process, all the substrates were ultrasonically cleaned in acetone and ethanol for 15 min respectively, and then mounted on holder at 10 cm in front of the targets. The substrate holder rotation speed was 3 rpm. Before the coating deposition, the chamber was firstly pumped down to a base pressure below 4×10-5 mbar, and the substrates were etched by Ar+ bombardments for 2 min with substrate bias voltages of -900 V, -1100 V and -1200 V separately to remove thin oxide layer and other adherent pollutants on the substrates surface. A 350 sccm gas flow of Ar was firstly used for the deposition of thin Cr interlayer from Cr targets using a 60 A target current and a −20 V substrate bias for 10 min. For the CrN layer, N2 was introduced into the chamber at 600 sccm gas flow. The target current of 60 A and the substrate bias of −20 V were applied for all coatings depositions. During the continuous growth of CrN coatings, the temperature of chamber and substrate was



maintained at 400 ℃ and the temperature deviation was lower than ±10℃. The different CrN layer deposition time with 1 h, 2 h, 5 h, 12 h, 18 h and 24 h were used for the CrN coatings growing up, which are named CrN(1h), CrN(2h), CrN(5h), CrN(12h), CrN(18h) and CrN(24h). More detailed deposition parameters were summarized in Table 1. 2.2. Microstructure characterizations The growth thickness, surface morphology and cross-sectional microstructure of the CrN coatings were characterized by the transmission electron microscopy (TEM, FEI Tecnai F20) and field-emission scanning electron microscope (FESEM, FEI QUANTA 250 FEG, USA and S-4800, Japan) equipped with EDS analyzer (OXFORD X - Max). The Laser confocal scanning microscope (Zeiss-material type, Germany) was employed to examine surface roughness (Ra) of coatings. The crystal structure of the coatings was determined by X-ray diffraction (XRD BRUKER D8 Advance, Germany) with Cu K-alpha radiation (wavelength = 0.154056 nm). The scanning angle was ranged from 20 to 90° at a scanning speed of 2°/min. Moreover, after using residual stress tester (JLCST022, Korea) to measure the curvature radii of the CrN coatings deposited on the Si substrate (length-width ratio ≥ 10), the residual internal stress changes with coating growth were calculated Stoney equation29-31. 2.3. Mechanical properties The elastic modulus and hardness of the as-deposited coatings were measured using a MTS Nano Indenter G200 system equipped with a Berkovich indenter. The coatings should be polished to surface roughness less than 50 nm before the tests to


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eliminate effects of the rough surface. The maximum indentation depth was 3000nm and 6 indentations on different areas of each sample were performed to acquire the mean value and the standard deviation. The hardness and elastic modulus were calculated according to the Oliver-Pharr method from the load-displacement curves as an average from 6 indents. The CSM Instruments Revetest with a conical diamond tip of 0.2 mm radius and 120° taper angle was used to determine the load bearing capacity of coatings and the adhesive and cohesive strength between coating and substrate during the scratch test. And the morphologies and microstructure of the scratch tracks were also observed in detail by scanning electron microscopy (SEM, FEI Quanta 250 FEG, USA) and focused ion beam instrument (FIB Carl Zeiss Auriga, German) to further determine the coatings failure mechanism. 2.4. Tribological properties Tribological property evolution with the growth of the CrN coatings was performed by a reciprocating ball-on-disk tribometer (CETR UMT−3MT, USA) under an air environment with room temperature of 20 ± 5 ℃ and relative humidity of 80 ± 5%. And the SiC balls, which have a hardness value of 24 GPa, were used as the counterparts with a diameter of 3 mm. A sliding stroke of 5 mm was applied in the experiments with the stroke frequency of 5 Hz, and the friction coefficient was continuously recorded during testing. The wear test cycle lasted for 60 min. The assessment of load bearing capacities under reciprocating sliding test was investigated with gradually increasing applied load in the form of gradient with 5 N for interval. The morphologies of the wear tracks on the coatings were examined using FESEM.



Based on the wear track depth profiles at several locations detected by an Alpha-Step IQ profilometer, the wear losses of the coatings V can be obtained after the sliding tests were completed. Then the formula K = V/FS was used to calculate the wear rate. V is the wear loss of coating in mm3, S is the total sliding distance in m and F is the normal load applied in N. 3. Results and discussion 3.1. Micro-structure and morphologies of the coatings


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Fig.1. Cross-sectional SEM micrographs, surface morphologies and the surface roughness of CrN coating with different deposition time: (a) CrN(1h), (b) CrN(2h), (c) CrN(5h), (d) CrN(12h), (e) CrN(18h), (f) CrN(24h), and (g) the EDS line scan of the CrN(24h) coatings along the thickness direction.

Fig.2. The high magnification cross-section of ultra-thick CrN coatings.

Fig. 1 shows the cross-sectional SEM micrographs of CrN coatings. The surface



morphologies and the surface roughness (Ra) of CrN coatings are input figures. As shown in Fig. 1a, many microcavities and spherical particles in different sizes are obviously observed on the surface of the CrN(1h) coating, which is feature of multi-arc ion plating during the deposition process. As increasing growth time, surface roughness (Ra) of coatings gradually increases and the macro particles on the coating surface increases and agglomerates simultaneously, resulting in the position of the craters are occupied and microcavities are gradually disappeared. From the cross-sectional SEM micrographs of as-deposited CrN coatings, the resulting total thicknesses of the CrN coating, with increasing continuous growth time, are determined to be 2.4, 5.4, 11.5, 41.5, 53.7 and 80.6 µm, respectively. The interlayer of Cr (400–500 nm) could be observed obviously between the CrN layer and substrate. The CrN layer show an obvious columnar feature comprised vertically aligned crystals for all CrN coatings. And it could be seen that CrN(1h) is the densest one among these coatings. As shown in Fig. 1 and 2, with the growth of coatings, the columnar crystals grow wider with forming “V” shape and the continuous growth of columnar crystals is interrupted and the some internal defects such as micro-pinholes and pores are gradually increased shown. The dense microstructure and fine crystallite size can be responsible for the excellent mechanical properties especially to the strength or hardness following the classical Hall-Petch relationship32. The cross-sectional line scan of the CrN(24h) coatings (Fig.1g) using the EDS analyzer clearly indicates the stable N/Cr ratios in the coatings. This shows that the chemical composition of the coatings is almost independent of the continuous growth of the


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coating during multi-Arc ion plating deposition process.

Fig.3. Typical bright field TEM images of the CrN coating with the increase of the growth time

Fig.4. The HRTEM observation of the CrN(5h) and CrN(24h) coating (a and b), with (c and d) corresponding inverse fourier-filtered images from inside the yellow box in (a and b) for a close-up view of the dislocations and stacking faults.

The cross-sectional TEM was used to investigate the changes in the



microstructure with the extension of the coating growth time. Compared with the bright field TEM images of as-deposited CrN structure with the increase of the growth time (Fig. 3), we notice that some fine cracks (indicated by arrows) are located bottom of columnar crystals of the CrN(5h) coating, which indicates the high internal stress generated from the accumulation of the defects. As the coating growth time becomes longer, cracks are significantly decreased and the columnar crystal structures are intact. The HRTEM observations (Fig. 4a) clarified the presence of some lattice distortions (yellow arrows) in the CrN (5h) coating, around which the stacking faults (pink box by dashes) and dislocations are also observed according to the inverse Fourier-filtered image (Fig. 3c)33-35. These defects are caused by the so-called ion-peening mechanism, kinetic neutral and column boundary interactions during deposition36, 37. In the CrN(24h) coating (Fig. 3b and d), however, the number of defects was much lower. This is mainly due to the relaxation mechanism which is the increase of coating continuous growth time and gradually accumulated energy make defects have more time to incorporate, diffuse, absorbed and annihilated finally11, 23, 36-39

. The absorption and annihilation of the defects play an important role to the

release of residual stress and decrease of CrN coating strength.


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Fig.5. XRD patterns of all CrN coatings with different deposition time (a) and the fractionated spectrum of CrN(24h) (b).

Fig. 5a shows the change of XRD patterns with CrN coatings growing thicker. All coatings exhibit face centered cubic (fcc) structure (JCPDS11-006540) with growing toward the CrN (200) (PDF 65-9001) orientation centered at 42.7°, which is due to the lower surface energy23, 41, 42. The intensity of the CrN (111) and Cr2N (110) peak are increased firstly with the coatings deposited time increasing to 2 h and then decreased. The CrN (220) peak is gradually appeared when the coating grows for more than 2 h. Further increasing the coating deposited time over 12 h, these phases are stable and have no obvious change. For the thin CrN coatings (thickness typically ≤10 µm), the peak positions both shifted toward a lower diffraction angle attributed to a higher residual stress in the coatings23, 43. Moreover, the broadened Cr2N (111) (PDF 27-0127) and Cr (110) (PDF 06-0694) phase diffraction peak are also observed at 2θ values of 42.6° and 44.4° for all coatings (Fig. 5b), which indicate that the as-deposited CrN coating mainly contains Cr, Cr2N and CrN phases. It is well known



that the crystallography structure of the CrN coating mainly depends on the N content in the chamber41. And the formation of Cr2N phase with the weaker intensity of Cr2N (111) peak may be attributed to the not high enough amount of nitrogen to acquire the stoichiometric CrN during the process of switching on the N2 flow, and the Cr2N was formed consequently44. The formation of Cr phase may be attributed to the droplet of Cr on the coating surface and the Cr interlayer which is especially for the relatively thin CrN coatings.

Fig.6. The residual stresses curve of CrN coatings with the increased growth time, and the digital images of test samples which is CrN coatings deposited on the Si substrate (length-width ratio ≥ 10) (the insets).

The residual stress and growth thickness of coating are plotted as a function of the growing time in Fig.6. The negative value means a compressive stress in the as-prepared coatings. As can be seen in Fig. 6, the residual compressive stress and CrN coating thickness both increase at beginning of coating growth. However, then


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coating thickness continuously increase while the residual compressive stress in the CrN coatings decreases rapidly from −0.62 GPa to −0.26 GPa when continuous growth time exceeds 5 hours. There are two main components to arise residual stresses. The first component is internal stresses generated in the CrN coating growth stage and the second component is due to thermal stresses which arise from the mismatched coefficient of thermal expansion (CTE) between coating and the substrate during the cooling period after the deposition15,

23, 36-39

. Since the relaxation

mechanism and the same deposition temperature for the CrN coating depositions, the residual stress gradually decreases as the growth time of coating further increased beyond 5 h. It is just because of the reduced residual stress that CrN coating can continuous to grow thicker and the coating thickness reach 80 µm at the growth time of 24 h. 3.2. Mechanical property and load bearing capacity

Fig.7. H/E and H3/E2 value evolution of CrN coatings with respect to the deposition time.

Hardness and elastic modulus are important properties for hard CrN coatings



affecting their resistance to wear. The evolution of hardness and elastic modulus for the CrN coatings with the increasing deposition time are given in Table 2. The CrN(1h) coating has the highest hardness and elastic modulus (29.14 and 436.62 GPa, respectively) among all the coatings. With the continuous growth of coating, the hardness and elastic modulus firstly rapidly decrease to 18.91 GPa and 306.19 GPa. When the growth time up to 12 hours or more, the hardness and elastic modulus of the CrN coating further dropped from 18.39 and 288.54 GPa to 14.723 and 256.72 GPa, respectively. It clearly demonstrates that the hardness and elastic modulus were decreased with the growth of the CrN coatings. The decreasing hardness of the coatings is mainly attributed to several reasons: one possibility is the crystallite coarsening with increasing growth time following the classical Hall–Petch relationship32, 45. And the other one is that the dislocations more quickly hit and are blocked at the interfaces between coating and substrate because of the short distance for thinner coatings23. The loose structure for the thick CrN coatings also can be responsible for the decreased hardness and elastic modulus. Moreover, J.M. Olaf et al. have verified that the influence of the compressive residual stresses on nanoindentation method is negligible by the FEM-calculations46. The H/E and H3/E2 ratios relate to the durability, elastic strain resistance and plastic deformation resistance of the coatings32, 47-50. It has been well known that the ability of CrN coatings to resist mechanical degradation and failure is improved by both high hardness (H) and low elastic modulus (E). The high H/E ratio well describes good ability of coatings to elastic deformation, which plays an important


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role in assessing the strength of the coatings on the local dynamic loads50, 51. The high H3/E2 imply high resistance to cracks initiation and propagation, which are also desirable for wear improvement11, 47, 52-54. The H/E and H3/E2 ratios evolution during CrN coating continuous growth are calculated shown in Fig. 7. The H/E and H3/E2 show a similarly evolutionary tendency with respect to the increasing deposition time. The CrN coating with deposition time of 12 h shows a desirable of 0.064 in H/E ratios and 0.075 in H3/E2 ratios, indicating a potential excellent wear resistance. The significant decreasing in the H3/E2 ratio when CrN coating growing time more than 12 h, which indicates decreased resistance to crack initiation and propagation.




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Fig.8. The scratch tracks and morphologies of CrN coatings with different deposition time: (a) CrN(1h), (b) CrN(2 h), (c) CrN(5 h), (d) CrN(12 h), (e) CrN(18 h), and (f) CrN(24 h).

The scratch-test is commonly used to evaluate the adhesive (detachment from coating / substrate interface) and cohesive (detachment taking place inside the coating) strength of coatings. The load bearing capacity of the coating can also be significantly reflected from scratch-test feedback for the different growing time of CrN coating. The high load bearing capacity of the coating also indicates a potential excellent wear resistance. The Fig. 8 presents scratch tracks of CrN coatings and SEM morphologies, which reveals the critical coating failure events43, 55. Although the thin Cr adhesion layer has been deposited at the same time as interlayer for all CrN coatings, the overall coatings exhibited different scratch behaviors. As shown in Fig. 8(a), for the CrN(1h) coating, the typical buckling failure appears at 23.4 N with the continuous chippings along the track and substrate is exposed finally with continuously increasing load, which indicate a poor adhesion of the coating. Combining with SEM micrographs of scratch tracks in Fig.8(b), the CrN(2h) coating with thickness of 5.4µm shows typical buckling failures at 26.1 N accompanied by curves cracks and



extensive cracking. And patches of damage are noted obviously at end of scratch track, which can be ascribed to the relatively high hardness of coating. It is clearly evident from Fig. 8(c) that the initial buckling failure of CrN(5h) coatings begin at 63.5 N by curved cracks and simultaneous chipping. And there are only circumferential cracks and plastic deformation at end of scratch track, which may be attributed to the decreasing hardness. When the coating continuous growing thicker, the critical load is significantly increased and the failure mechanism of the coating - substrate system also transforms. It is clearly evident from Fig. 8(d) that the wedge spallation failure of the CrN(12h) coating occurs at very high critical loads of 128.0 N and the initial cracks begin at a high critical load of around 72.5 N. The cracks which noted in the scratch track stopped at the interface of CrN coating seen in the FIB cross-sections. Combine with the cross-sectional profiles of scratch tracks, thickness of wedge spallation is less than that of the CrN(12h) coating, which shows the coating is still partly adhered to the substrate and CrN(12h) coating occurs cohesive failure. The high load bearing capacity and improved cohesive strength for CrN(12h) coating with thickness of 41.5 µm can be primarily attributed to the desirable combination of high H/E and H3/E2 ratios and a lower modulus49, 56. The appropriate residual compressive stress, which act as so-called “pre-stress” in the CrN(12h) coating, can effective diminish coating cracking failure due to working tensile stress [47]. Similarly, the wedge spallation failure occurs at further high critical loads of 163.3 N for the CrN(18h) coating, which indicates further increasing load bearing capacity. But the initiation of cracks occurs at lower critical load than that in CrN(12h) coating. This is


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due to the decreasing residual compressive stress is unable to compensate for tensile stresses arising from the working load. When the CrN coating continued to grow for 24 h, it exhibits further increasing load bearing capacity and reducing cohesive strength. As shown in Fig. 8(f), the nature of wedge spallation failure cannot be seen at the end of the scratch track. Obviously, there are only the cracks which initially appear in the scratch track at a low critical load of 62.3 N, and FIB cross-section shown in Fig. 8(f) shows the cracks only extend around 10 µm and stop at the interface of CrN coating. The low cohesive for the CrN(24h) coating is mainly due to the lowest residual compressive stress and lowest H/E and H3/E2 ratios among the coatings. From the present study, it can be seen that cohesive failure becomes the dominant failure mechanism with wedge spallation and cracking failure during scratch test for the ultra-thick CrN coating, which is obviously different from the traditional thin coating (thickness typically ≤10 µm) with buckling failure bending in response to high compressive stresses36, 57. The small thickness and relatively high compressive stresses in coatings could decrease the critical load during scratch-test. And greater thickness of coating, which can consume more energy during the process of crack propagation and spallation from surface to substrate, as well as the reduced compressive stresses contribute together to the significantly improved load bearing capacity as the CrN coating growing57, 58. Moreover, low compressive stresses and decreasing H/E and H3/E2 ratios also can contribute to reduce resistance to crack initiation and propagation for the ultra-thick CrN coatings.



3.3. Tribological behaviour and load bearing capacity The Table 3 presents the some values for the load bearing capacities of the coatings during reciprocating sliding test. For CrN(1h) coating with a thickness of 2.4 µm, the load bearing capacity is worst and it is worn through under the load of 10N (3.6 GPa). As the coatings growing thicker, the CrN(2h) and CrN(5h) coating are failure when the load is increased to 25 N and 38 N, respectively. And when the coating growth time exceeds 12 hours (thickness ≥ 40 µm), the values for the load bearing capacity exceed 40 N even though the contact pressure is 4.9 GPa, which indicates ultra-thick CrN coatings have the excellent load bearing capacity compared with the thin coatings.

Fig.9. Average friction coefficients and wear rates of CrN coatings with different thickness sliding against SiC balls in ambient air.


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Fig.10. SEM micrographs of wear track in ambient air.

Fig. 9 shows the evolutions of average coefficient of friction (COF) and wear rate as the CrN coatings growing thicker under the load of 10N in ambient air. As shown in Fig. 9, the average COF values firstly increases to 0.447 when the coatings growth for 5 h and then gradually decreases to 0.37 as the coatings continuing growth, even though surface roughness gradually increasing. The CrN(2h) coating with the thickness of 5.4 µm shows highest wear rate of 11.6 × 10−7 mm3/(Nm) although it exhibit relatively high hardness. This can be attributed to the inferior adhesive strength and low load bearing capacity. As the coatings continuously growing thicker, the wear rate significantly decreases and then coatings show a stable wear resistance at a minimum value of 5 × 10−7 mm3/ (Nm) when the coating growth time exceeds 5 hours. The mean coefficient of friction and wear rate of thicker CrN coating are respectively decreased by 17.2% and 56.8% at most compared with the thin CrN(2h) coating. The SEM images of wear track morphology are presented in Fig. 10. The



worn surface of CrN(2h) coatings reveals a smooth simultaneously accompanied furrows paralleled to the sliding direction and the peeling strips in the middle of the wear tracks. It clearly indicates that thin CrN coatings failure is controlled by the abrasive wear mechanism. As the coatings grow thicker, it can be seen obvious change for wear topography on coatings surface especially when coatings continuous growth time up to 12 h or more. Due to the decrease of hardness, the phenomenon of furrows obviously disappears for the ultra-thick coatings (deposition time ≥ 12 h). Moreover, the obvious presence of the desquamation, spallation and large micro cracks on the wear tracks can be noted, which is attributed to the increased roughness of coatings and decreased hardness make wear debris cannot be discharged effectively and constantly rolled and fragmented as the third - body layer during the friction test. The softer third - body layer can participate in the interface behavior and bearing to improve the load bearing capacity of coating, which improve the wear resistance of ultra-thick CrN coating44.

Fig.11. The model of Hertzian contact between SiC ball and CrN coating


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According to the Hertz solution59-61, the contact pressure of the compressed coatings is distributed in a planes-strain model (Fig. 11). The parameter F represents normal load and R represents counterpart ball radius. The maximum contact pressure p0 and the contact radius a, are written as 0 =

(1)

(2)

2

a= E* =

*

12 1

+

22

(3)

2

Where the E* represents equivalent elastic modulus, ϑ1 , ϑ2 and E1, E2 represents the Poisson’s ratio and Young modulus of the SiC ball and coating, respectively. The stress field along the depth directions of coating (z-axis) can be expressed as follows:

σ = −p ∗ + 1! "#

%$

= 1 + !

(4)

− &1 + ϑ' (1 − tan !+

σ, = σ

(5) (6)

where σ , σ, and σ are the normal stresses in radial, circumferential and depth directions, respectively. Combining equation (1), (2) and (3), the maximum contact pressure stress of the CrN coatings is decreased and the position of the maximum stress is simultaneously moved into the coating interface as the coating thickness increases, which have been identified by the many researchers using FEM simulation8, 62, 63. And according to the von Mises yield criterion59, 61 and combining equation (4), (5) and (6), the stress (/01 )



Page 26 of 32

at any location on the depth directions (z-axis) can be expressed as follows: 234 5%

= 1 + !

− &1 + ϑ' (1 − tan !+

(7)

It has been reported that ϑ for the CrN coating is 0.25. Hence the maximum

depth of contact stress is 19.35 µm under the load of 10N, which is bigger than thickness of CrN(5h) coatings, but much smaller than the thickness of CrN(12h) coatings. Therefore, the ultra-thick CrN coating can provide greater stress shielding for the substrate and protects the substrate from wear penetration (inset of the Fig. 11). Conversely, the protective effect of ultra-thick CrN coatings could get rid of the influence of counterforce generated from substrates plastic deformation during the dry sliding. Furthermore, the good cohesive strength and load bearing capacity for gradually thicken coating as CrN continuously growing could effectively prevent coating wear during the dry sliding. Hence, on the basis of the above several points, the ultra-thick CrN coatings present excellent wear resistance in ambient air. 4. Conclusions From what discussed above, the following conclusions can be drawn: (1) Since the relaxation mechanism results in the gradually decreased residual stress as the CrN coating growth time exceeds 5 hours, continuous growth of traditional monolayer CrN coatings up to 24 h could be successfully achieved to fabricate ultra-thick coating (up to 80.6 µm) on the 316 stainless steel substrate using multi-Arc ion plating technique. (2) During the scratch tests, the ultra-thick CrN coatings show good cohesive strength and significantly improved load bearing capacity compared with traditional thin


Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coating, which is mainly attributed to the decreasing residual stress and increasing thickness. And the cohesive failure (wedge spallation and cracking failure) becomes the dominant failure mechanism during scratch test for the ultra-thick CrN coating. (3) The sliding-friction test also clearly revealed the excellent load bearing capacity of ultra-thick CrN coating (deposition time ≥ 12 h). As the thickness increases, the CrN coating can provide greater stress shielding for the substrate with the decreased maximum contact pressure stress and contact stress field was limited totally in the coating interface. Therefore, the ultra-thick CrN coatings present low friction and excellent wear resistance in ambient air. This simple route to achieve low friction, low wear, and high load-bearing capacity for CrN coating is desirable for the fundamental researches and commercially industrial applications in potential engineering.

Acknowledgments

The authors gratefully acknowledge the financial support from the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA13040602), the National Basic Research Program of China (973 Program) (Grant no. 2014CB643302), the National Natural Science Foundation of China (Grant no. 51475449) and the Key research and development program of Jiangsu Province (Grant no. BE2016115).



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Page 32 of 32

Table of Content Graphic: Table 1 The deposition conditions for the Cr interlayer and CrN layers. Bias

Target

Chamber

Partial

Partial

Deposition

voltage (V)

current (A)

temperature

pressure of

pressure of

time

(℃)

N2 (mbar)

Ar (mbar)

Cr layer

-20

60

400

-

1.2∗10-2

10 min

CrN layer

-20

60

400

5∗10-2

-

1-24 h

Table 2 Thickness，hardness and elastic modulus of the coatings Samples

Deposition

Coating

Coating

Hardness

Elastic modulus

time (h)

thickness

roughness

(GPa)

(GPa)

(µm)

(µm)

CrN(1h)

1

2.4

0.27

29.14 (±1.48)

436.62 (±24.41)

CrN(2h)

2

5.4

0.35

19.61 (±1.45)

328.19 (±27.30)

CrN(5h)

5

11.5

0.75

18.91 (±2.16)

306.19 (±18.31)

CrN(12h)

12

41. 5

1.03

18.39 (±2.23)

288.54 (±13.10)

CrN(18h)

18

53.7

1.14

17.64 (±2.87)

284.61 (±16.53)

CrN(24h)

24

80.6

1.28

14.72 (±2.06)

256.72 (±22.81)

Table 3 Some values for the load bearing capacities of the coatings Sample Load bearing capacity (N) Contact pressure (GPa)

CrN-1h

CrN-2h

CrN-5h

CrN-12h

CrN-18h

CrN-24h

10N

25N

38N

≥40N

≥40N

≥40N

3.6

4.4

4.9

4.9

4.9

4.7


Continuously Growing Ultrathick CrN Coating to Achieve High Load

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